1. Field of the Invention
The present invention relates to a photodiode, and particularly to a structure for improving the short wavelength region photo sensitivity of a photodiode, and to a method of manufacturing the photodiode.
2. Description of the Related Art
Conventionally, a photodiode is used as an optical detection element, for example, as a signal detection element of an optical pickup. In recent years, because of an increase in capacity of an optical disk, there has been proposed optical disk readout by a short wavelength semiconductor laser with blue light or violet light. Thus, the short wavelength region photo sensitivity of the photodiode becomes very important.
FIG. 2 is a sectional view of a conventional photodiode.
In FIG. 2, the photodiode is constituted by a first conductive type semiconductor region 1, for example, an N-type silicon semiconductor substrate, and a second conductive type semiconductor layer 2 formed on the surface of the first conductive type semiconductor region 1, for example, a P-type diffusion layer. An antireflection film 5, for example, an oxide film is formed on the photodiode. A reverse bias is applied to the first conductive type semiconductor region 1 and the second conductive type semiconductor layer 2, and when light is irradiated to a depletion layer 3 formed by the reverse bias application, an optical signal is generated in the depletion layer 3.
The depth of the second conductive type semiconductor layer 2 of the photodiode in FIG. 2 is optimized in accordance with a wavelength to be detected, and for example, in the case where red light of a wavelength of about 800 nm is detected, the depth of the second conductive type semiconductor layer 2 is made about 3 xcexcm. In the case where blue or violet light of a wavelength of about 400 nm is detected, light of a wavelength of about 400 nm is easily absorbed by silicon and is almost absorbed in a depth of about 1 xcexcm. Thus, in order to detect short wavelength light of a wavelength of about 400 nm in a conventional photodiode, it is necessary that the depth of the second conductive type semiconductor layer 2 is made 1 xcexcm or less. However, in the conventional photodiode, since it is difficult to control the depth of the second conductive type semiconductor layer 2 so that the depth becomes 1 xcexcm or less, there has been a problem that light sensitivity in a short wavelength region is inferior.
If a plurality of second conductive type semiconductor layers 2 are formed as in a photodiode shown in FIG. 3, a depletion layer 3 is formed on the surface of a first conductive type semiconductor region between the second conductive type semiconductor layers 2a and 2b, so that the photo sensitivity in a short wavelength region is improved. However, an interface level 4 is formed on the surface of the first conductive type semiconductor region between the second conductive type semiconductor layers 2a and 2b, and an optical signal generated in the depletion layer 3 between the second conductive type semiconductor layers 2a and 2b is trapped by the interface level 4, so that there has been a problem that the photo sensitivity is lowered and leak current is increased.
The present invention has been made in view of the foregoing drawbacks in the conventional art, and has an object to provide a photodiode which can improve the photo sensitivity in a shorter wavelength region, as compared with a conventional photodiode, without increasing leak current.
In order to solve the above-mentioned problem, according to the present invention, a photodiode includes a first conductive type semiconductor region, and a plurality of second conductive type semiconductor layers formed on the surface of the first conductive type semiconductor region, the first conductive type semiconductor region and the plurality of second conductive type semiconductor layers constituting an optical detection portion for detecting an optical signal and outputting its photoelectric conversion signal, wherein the surface of the first conductive type semiconductor region between the second conductive type semiconductor layers is removed. By this construction, since the surface of the first conductive type semiconductor region between the second conductive type semiconductor layers is removed, the interface level of the surface of the first conductive type semiconductor region between the second conductive type semiconductor layers is removed.
Furthermore, the surface of the first conductive type semiconductor region is removed by a wet etching method, so that the region can be removed without generating an interface level due to etching damage.
In the photodiode of the present invention, the plurality of second conductive type semiconductor layers are formed on the surface of the first conductive type semiconductor region.
When a reverse bias is applied to the photodiode constituted by the first conductive type semiconductor region and the second conductive type semiconductor layers, a depletion layer is extended in accordance with the bias voltage. Since the depletion layer is extended not only in the vertical direction but also in the horizontal direction, the depletion layer is extended not only in the vertical direction but also in the horizontal direction, the depletion layer is also formed on the surface of the first conductive type semiconductor region, and the photo sensitivity in a short wavelength region is improved. At this time, the respective second conductive type semiconductor layers are made to have the same potential. Thus, when the distance between the second conductive type semiconductor layers is made about twice the width of the depletion layer, since the depletion layers of adjacent photodiodes formed of the first conductive type semiconductor region and the second conductive type semiconductor layers just come in contact with each other, the efficiency is excellent.
Further, in the first conductive type semiconductor region between the second conductive type semiconductor layers, the interface level is formed on the surface of the first conductive type semiconductor region, and by removing the interface level, the leak current can be controlled without greatly influencing the photo sensitivity of the photodiode.
Furthermore, when the interface level of the surface of the first conductive type semiconductor region is removed by using the wet etching method, the interface level can be removed without generating an interface level due to etching damage.